Thermal conditioning for electrostatic



THERMAL CONDITIONING FOR ELECTROSTATIC SEPARATION OF GET Foster Fraas,Hyattsville, Md., assignor to the United States of America asrepresented by the Solicitor of the Department of the Interior NoDrawing. Application December 1, 1952, Serial No. 323,524

7 Claims. (Cl. 209-11) (Granted under Title 35, U. S. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the Government of the United States for governmental purposeswithout the payment to me of any royalty thereon in accordance with theprovisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

This invention relates to mineral separation and more particularly tothe electrostatic separation of garnet from associated minerals.

Garnet found in mineral mixtures is frequently difficult to separatefrom the other minerals by the common methods of gravity, frothflotation, and magnetic separation. Electrostatic separation may beemployed Where the more common methods of separation are notpracticable. It has been found, however, that many forms of garnet donot possess suflicient electrical conductivity to be selectivelyseparated from nonconductive associated minerals by this method.

Accordingly, an object of this invention is to provide a method forincreasing the selectivity of the electrostatic separation of garnetfrom other minerals.

Another object of this invention is to provide a method for increasingthe electrical conductivity of certain forms of garnet whereby thegarnet may be selectively separated by electrostatic methods fromassociated nonconducting mineral.

Another object of this invention is to provide a method whereby certainforms of e garnet may be selectively separated from both conductive andnonconductive associated minerals by electrostatic methods.

These and other objects and advantages, hereinafter apparent as theensuing description proceeds, are accomplished by this invention whichcomprises thermally conditioning mineral mixtures containingnonconductive forms of garnet to render the garnet conductive prior tothe electrostatic separation of garnet from other nonconductingmaterial. An electrostatic separation of this form of garnet from bothconducting and nonconducting material may be accomplished by firsteiectrostatically separating the conducting material from thenonconducting fraction, thermally conditioning the nonconducting nitedStates PatentO 1y all of the alkaline earth metal or aluminum, or bothalkaline earth metal and aluminum have been replaced by an element ofthe iron group, namely, Fe, Mn, Cr, or Ni. The presence of an iron groupelement in the mineral structure contributes to the conductingproperties of the mineral. For example, in the complete absence of aniron-group element a conductive structure would not be possible,whereas, in most instances a mineral high in iron-group element contentpossesses a conductive structure and exhibits the properties of a goodconductor upon electrostatic separation.

It has been discovered that many garnet minerals which normally are pooror nonconductors, nevertheless, contain sufficient iron-group elementsin such form that they may be converted to conductors by heating to asuitable transition temperature. These garnets include those, such asgrossularite, (CaO)3( /2Al2O3)23SiO2 and pyrope (MgO)3( /2Al2O3)z3SiOz,for which the theoretical formula indicates no iron group content.Elements of this group do occur, however, in all but the most rarecolorless forms of garnet, at least to such an extent that an increasein conductivity takes place upon heat treatment. Other forms of garnetwhich are rendered more highly conductive upon thermal treatmentinclude.

rhodolite, uvarovite, and spessartite. While the invention is not to belimited by any theory of chemical or physical transition, it wouldappear that a crystal change occurs during heating which results inrearrangement of the iron group elements to a form commonly found insemiconductor crystal structure. That such a change apparently occursmay be noted by visual observation. For example, a garnet before heattreatment may have the transparency of common glass or of quartz orruby. After heat treatment, and cooling to the original temperature, thegarnet is still transparent but has acquired the metallic reflectivityof semi-conductors such as hematite, cuprite, and andradite.

The data of Table I shows the effect of thermal treatment upon a numberof mineral varieties of garnet. Almandite and the higher-coloredandradites are normally good conductors and require no thermaltreatment. Two sources of andradite which are insulators beforetreatment have no change after treatment. Of the andradites they are thelightest in color and their iron group elements are evidently notavailable for a conductivity increase after thermal treatment.

Table I.Thermal conditioning of garnet Conditiong Effeet VarietyTheoretical Formula Source Color Before After Rhodolite (FeO,MgO)a(%A120a)23Si0g Jackson County, N. C. pink I O1 (MgO) A1203 23Si02 NavajoRes, Ariz bluish red I O2 (CaO);( CrgOQzBSiOr Orford, Quebec green I 01(0210).;(Vg A]zO3)23SiO-z Minas Gerais, Brazil. blOWIllSh red I O2(CaO)3( A1203)z3si()z Reddington, Conn grayish orange. I C2 (M)A1203)23Si0z Madagascar I O2 (FeO) Al2Oa)23SiOz Gore Mtn., N. Y C1(OaO)a(% Fe Os)z3SiO2 Willsboro, N. Y d 01 (GaO) F8203)Z3Si02 Ala,Piedmont, Italy yellow I I (GaOhU/z F6203)3Si0z Graham County, Ariz.greenish yellow. G1 (OaOhOA Fe2O3)z3SiO2 Franklin, N. J yellow I I(CaO)a(% F8z03)z3$i02 Oslo, Norway black O1 I=insulator; C1=goodconductor; C2=poor conductor.

The samples in Table I were conditioned by heating at 650 C. for fifteenminutes, cooling and testing for conductivity. The conductivity testconsisted of placing the mineral particle between the terminals of anelectrostatic voltmeter of 82 mmf. capacity charged to 280 ,volts. Withminerals designated as insulators, there was no drop in potential duringa one minute contact period. With minerals designated as poorconductors, C2, the potential dropped approximately 40 volts in thirtyseconds, and with good conductors, Cl, much more rapid- .ly. Themoisture content of the surrounding air was 34 per cent relativehumidity. This is sutriciently low to preclude any surface conductivitydue to moisture films. Although heating under reducing conditions mayyield higher conductivities, it is a more expensive operation than theabove simple procedure of heating in air.

A temperature of about 650 C. is preferred for the :thermal treatment.Lower temperatures can be used but the transition rate of the garnet toelectrically conductive forms decreases as the temperature is lowered.At 650 C. the transition rate is sufficiently rapid for practicaloperation. The mineral mixture is then cooled to the customarytemperature for electrostatic separation. A vtemperature between aboutC. to 100 C. is suitable for this operation.

The electrostatic separation may be secured with any electrostaticseparator which separates on the basis of difference in conductance. Thedielectrode type of roll separator has been successfully utilized forthis purpose.

The invention will be further illustrated by the following examples ofpractice.

EXAMPLE I This application of the process utilized a North Carolinamonazite rich sand containing the commonly associated minerals ofgarnet, ilmenite and rutile. The garnet was suiiiciently low in the irongroup elements so as to respond as a nonconductor but had a suflicientcontent of the iron group elements so as to have the same magneticsusceptibility as the monazite by magnetic methods. The ilmenite andrutile respond as good conductors.

The monazite material was passed through an electrostatic separator. Thegarnet and monazite collected together as nonconductors while theilmenite and rutile collected as conductors. The nonconductive fraction,of garnet and monazite, was then given a thermal treatment at about 650C. for about fifteen minutes. After cooling to room temperature, thisfraction was again passed through the electrostatic separator. aratelycollected as a conductor from the monazite as a nonconductor. Theresults of the separation are summarized in the following tabulation:

Table Il.-Efiect of thermal conditioning an electrostatic separation Theilmenite and rutile fraction collected before the thermal treatment isdesignated as product 1. The garnet separated as a conductor afterthermal treatment is designated as product 2, and the remaining monaziteis designated as product 3. It will be seen from the results in Table 2that the separated products 1 and 2, while making up a substantial partof the total sample on a weight The garnet now sep- 4 per cent basis,contained a very small amount of the radioactive monazite. Thus, ahighly selective separation has been effected.

EXAMPLE II Monazite sand is deslimed and sized by hydraulicclassitication to yield two sized fractions, minus 20 plus 35-mesh, andminus 35-mesh. Each fraction is gravity separated on a shaking wet tableto remove the quartz as reject. The high density concentrate containingilmenite, rutile, garnet and monazite is dried at the customary dryingtemperature of to C. and .passed through the electrostatic separatortoremove the ilmenite and rutile as conductors from the garnet'andmonazite as nonconductors.

The garnet and monazite is .now heated to the garnet transitiontemperature. At 650 C. the rate is sufliciently rapid. After cooling themixture to a temperature below 100 C., the electrostatic separation isrepeated to remove the garnet as a conductor from the monazite as anonconductor. The garnet in this ore was in the form ,of rhodolite.Rhodolite has a theoretical'formula intermediate between pyrope andalmandite, part of the MgO in pyrope being replaced 'by 'FeO. Thisparticular ore-- also contained xenotime which separates with themonazite. Xenotime is similar to monazite since it is composed of therare earth'elements. the substitution of a small amount of uranium for asmall amount of thorium.

The combined results on the minus 20 plus SS-mesh and the minus BS-meshfractions are summarized in Table III.

Table IIL-Gravity and --electr0static concentration of monazite sandProduct Anal Distriys s $835 Weight, Geiger g m; o1 Efiect Percentactivity, ass Percent rutile-ilconductor 22. 2 20 0. 3

menite. before. garnet... conductor 15.6 41 0.4

after. monazitenoneonduc- 36. 5 4, 332 97.1

xenotime. tor after. quargz re- 5. 8 69 0. 2 e

quartz re- 19.9 162 2. 0 ject 2. 100 1, 628 100 No complete analysis wasmade on this ore. The monazite-xenotime fraction 'by chemical analysiswas found to contain 53.7 per cent rare earth oxides. Since monazite andxenotime are the only radioactive minerals in this ore, the Geiger countshows the amount of these minerals in the other fractions. Thus, ahighly selective separation has again been made.

In addition to monazitesand, this methodmay be used wherever anonconductor fraction occurs in the concentration of garnet ores.Examples are mixtures of diamond and garnet and of kyanite and garnet.

It will be appreciated from a reading of the foregoing specificationthat the invention herein described is suseeptible of various changesand modifications without departing from the spirit and scope thereof.

What is claimed is:

1. A method of conditioning garnet containing an iron-group element andhaving a mineral form selected from the class consistingof rhodolite,pyrope, uvarovite, grossularite, and spessarite, for electrostaticseparation comprising thermally treating said garnet to increase theelectrical conductivity thereof.

2. A method of conditioning garnet containing an iron-group element andhaving a mineral form selected from the class consistingof rhodolite,pyrope, uvarovite, grossularite, and spessarite, for electrostaticseparation The difference is in comprising heating said minerals to atemperature at which the garnet becomes electrically conductive.

3. A method of conditioning garnet containing an iron-group element andhaving a mineral form selected from the class consisting of rhodolite,pyrope, uvarovite, grossularite, and spessarite, for electrostaticseparation, comprising heating said minerals to a temperature of about650 C. to increase the electrical conductivity of said garnet.

4. A method for the electrostatic separation of garnet containing aniron-group element and having a mineral form selected from the classconsisting of rhodolite, pyrope, uvarovite, grossularite, andspessarite, from associated minerals which respond as nonconductors inthe electrostatic process comprising heat-treating said minerals at atemperature at which the garnet becomes electrically conductive, andelectrostatically separating the conductive garnet from thenonconductive minerals.

5. A method for the electrostatic separation of garnet containing aniron-group element and having a mineral form selected from the classconsisting of rhodolite, pyrope, uvarovite, grossularite, andspessarite, from associated conductive and nonconductive mineralscomprising electrostatically separating the conductive minerals from thenonconductive minerals, heating said nonconductive mineral fraction toconvert the garnet contained therein to a conductive material, andsubjecting the heat-treated material to further electrostatic separativeto separate the garnet from the remaining nonconductive material.

6. A method for the separation of garnet from monazite rich sandcontaining rutile, ilmenite, and garnet containing an iron-group elementand having a mineral form selected from the class consisting ofrhodolite, pyrope, uvarovite, grossularite, and spessarite, comprisingelectrostatically separating the rutile and ilmenite from monazite andgarnet, heating the monazite-garnet fraction to a temperature of about650 C., cooling the heated material, and electrostaticaly separating thegarnet from the monazite.

7. A method for the separation of garnet from monazite rich sandcontaining rutile, ilmenite, and garnet containing an iron-group elementand having a mineral form selected from the class consisting ofrhodolite, pyrope, uvarovite, grossularite, and spessarite, comprisingsubjecting said sand to electrostatic separation whereby a conductivefraction containing rutile and ilmenite and a nonconductive fractioncontaining monazite and garnet are obtained, heat-treating saidmonazite-garnet fraction at a temperature of about 650 C. to convert thegarnet to a conductive form, cooling the heat-treated material to atemperature below 100 C., and subjecting this material to a furtherelectrostatic separation to separate the conductive garnet from thenonconductive monazite.

References Cited in the file of this patent UNITED STATES PATENTS959,646 Swart May 31, 1910 1,679,739 Overstrom Aug. 7, 1928 2,180,804Fahrenwald et al Nov. 21, 1939

1. A METHOD OF CONDITIONING GARNET CONTAINING AN IRON-GROUP ELEMENT ANDHAVING A MINERAL FORM SELECTED FROM THE CLASS CONSISTING OF RHODOLITE,PYROPE, UVAROVITE, GROSSULARITE, AND SPESSARITE, FOR ELECTROSTATICSEPARATION COMPRISING THERMALLY TREATING SAID GARNET TO INCREASE THEELECTRICAL CONDUCTIVITY THEREOF.